In recent years, communication devices provided with various communication functions have been developed. The communication devices transmit and receive various types of data to and from other communication devices that are individually connected to those communication devices.
Among these communication devices that transmit and receive these types of data, there arises a difference between the time at which data were transmitted from one communication device (hereinafter referred to as the transmission device) and the time at which the data were received by another communication device (hereinafter referred to as the reception device). This is caused by both a delay time of transmitted data due to a constant transmission delay that takes place over the transmission path and a delay time due to a variable load delay depending on the traffic and so forth of the network.
In the case in which a reception timing of data has been specified in the reception device, when the data are transmitted from the transmission device to the reception device, it is necessary to transmit the data by taking into consideration these delay times. For instance, in the case in which particular data need to be transmitted from a transmission device to a plurality of reception devices and the data need to be received simultaneously by the plurality of reception devices, it is necessary to consider delay times between the transmission device and each of the plurality of reception devices. In this case, the timings at which data are transmitted to individual reception devices are adjusted by the transmission device depending on the individual delay times that take place between the transmission device and each of the reception devices.
FIG. 1 is a schematic diagram showing one aspect of an ordinary communication system.
The communication system shown in FIG. 1 is structured such that a plurality of communication devices 1001-1 to 1001-4 are mutually connected through network 1002.
Communication devices 1001-1 to 1001-4 are devices connected to network 1002 and individually have a communication function that can mutually transmit and receive data thereamong through network 1002.
In the communication system as shown in FIG. 1 for instance, in the case in which the delay time of data is different between communication devices 1001-1 and each of communication device 1001-2 to 1001-4, when data are transmitted from communication device 1001-1 such that the data are simultaneously received by communication devices 1001-2 to 1001-4, it is necessary to decide the transmission timings at which the data are transmitted from communication device 1001-1 to transmission devices 1001-2 to 1001-4.
FIG. 2 is a schematic diagram showing the timings at which data are transmitted from communication device 1001-1 to communication devices 1001-2 to 1001-4 in the communication system shown in FIG. 1. In this instance, the case in which the delay time of data between communication device 1001-1 and communication device 1001-2 is 100 ms; the delay time of data between communication device 1001-1 and communication device 1001-3 is 400 ms; and the delay time of data between communication device 1001-1 and communication device 1001-4 is 500, will be described. The timing diagram shown in FIG. 2 is a timing diagram showing that data transmitted from communication device 1001-1 are received simultaneously by communication devices 1001-2 to 1001-4.
First of all, data are transmitted from communication device 1001-1 to communication device 1001-4 that has the largest delay time of 500 ms.
Then, another 100 ms later, data are transmitted from communication device 1001-1 to communication device 1001-3 that has the second largest delay time of 400 ms. Then, another 300 ms later, data are transmitted from communication device 1001-1 to communication device 1001-2 that has the smallest delay time of 100 ms.
Communication device 1001-4 receives data 500 ms after the data are transmitted from communication device 1001-1 to communication device 1001-4.
On the other hand, communication device 1001-3 receives data 400 ms after the data are transmitted from communication device 1001-1 to communication device 1001-3.
On the other hand, communication device 1001-2 receives data 100 ms after the data are transmitted from communication device 1000-1 to communication device 1001-2.
In other words, since data are transmitted to communication devices 1001-2 to 1001-4 at transmission timings corresponding to individual delay times, the data are received simultaneously by communication devices 1001-2 to 1001-4.
Thus, in the case in which data are transmitted from communication device 1001-1 such that communication devices 1001-2 to 1001-4 simultaneously receive the data, the individual delay times of data become critical factors.
Such a delay time, as described above, is composed of a constant delay time due to a constant transmission delay and a variable delay time due to a variable load delay depending on the traffic and so forth of the network. Their total time becomes a delay time.
Constant delay times just depend on the distances between communication devices 1001-1 and each of communication devices 1001-2 to 1001-4 and the transmission medium for use.
On the other hand, since variable delay times variably change depending on the traffic and so forth of the network, their mean value may be used as the delay time (for example, refer to JP2003-283374A).
Here, the above-described delay time computation method will be described.
FIG. 3 is a timing diagram showing the case in which measurement packet data, with which the delay time from a transmission device to a reception device is measured, is transmitted. In this case, the transmission device is referred to as the Time Master (PTP Server); the reception device is referred to as the Time Slave (PTP Client).
Measurement packet data that were transmitted at a time to from an application that operates in the transmission device were transmitted from a PHY layer to the reception device at a time t1. At this time, information that represents the time to is added to measurement packet data that are transmitted from the transmission device to the reception device. In addition, the application is informed of the time t1 at which the measurement packet data were transmitted from the PHY layer such that an inner delay time A of the transmission device is computed by the application.A=(t1−to)
Thereafter, the computed delay time “A” is transmitted to the reception device.
Then, the measurement packet data transmitted from the transmission device are received at a time t2 by a PHY layer of the reception device. Thereafter, the measurement packet data received at the time t2 by the PHY layer are received at a time t3 by the application. Then, the application is informed of the time t2 at which the measurement packet data are received by the PHY layer from the PHY layer to the application.
At this point, the application of the reception device computes a delay time C of the measurement packet data transmitted from the transmission device to the reception device.C=t3−t0 
Thereafter, the application of the reception device computes a compensation delay time B of the measurement packet data transmitted from the transmission device to the reception device.B=C−A−(t3−t2)
Here, the compensation delay time is the time after the measurement packet data are transmitted from the transmission device until they are received by the reception device and is a delay time due to external factors not caused by the transmission device and the reception device.
Then, likewise, the application of the transmission device computes a compensation delay time D of the measurement packet data transmitted from the reception device to the transmission device.
With the compensation delay times B and D computed in such a manner, the delay time of the measurement packet data between the transmission device and the reception device is computed.
When the delay time is actually computed, the above-described computations are performed for a predetermined period of time (number of times) and the mean value of values obtained by the computations is calculated so as to obtain the means value as a delay time.
However, when the mean value of delay times is computed, unless the delay times measured during a predetermined period are stable to some extent, if the delay times are unstable (largely fluctuate) due to the state of the transmission path (fluctuation of traffic), the accuracy degrades.
Thus, the problem occurs in which, when an attempt is made to improve accuracy, the period that is required to measure time is likely to become longer, resulting in prolonging the amount of time that is need to collect sample periods of time that are necessary for computing the mean delay time.
FIG. 4 is a graph showing a temporal change of delay times in the case where they are stable.
As shown in FIG. 4, when delay times are stable, they are nearly constant as time passes. Thus, delay times do not largely differ depending on the timings at which measurement packets, with which delay times are measured, are transmitted.
FIG. 5 is a graph showing relationships between delay times and their occurrence frequencies in the case where the delay times are stable as shown in FIG. 4.
As shown in FIG. 5, in the case where delay times are stable, a profile that indicates which measured delay times concentrate at a particular time is obtained.
FIG. 6 is a graph showing a temporal change of delay times in the case where they fluctuate.
As shown in FIG. 6, in the case where delay times fluctuate, delay times are not temporally constant as time passes. Thus, delay times largely differ depending on the timings at which measurement packets, with which delay times are measured, are transmitted.
FIG. 7 is a graph showing relationships between delay times and their occurrence frequencies in the case where the delay times fluctuate.
In the case where delay times fluctuate as shown in FIG. 7, a profile that indicates which measured delay times spread is obtained. To compute the mean value of the spread delay times without sacrificing accuracy, the delay times need to be measured for a period of time longer than the case in which they are stable.